1. Field of the Invention
This invention relates to the treatment of a hot aqueous brine solution containing various dissolved components, such as iron, silver and silica, to inhibit precipitation of undesirable scale, such as iron silicate scale, while enhancing deposition and recovery of a valuable silver-containing scale. More particularly, the invention relates to such a treatment wherein the scale is formed in a geothermal brine which is being processed to recover its heat content and the brine is handled prior to injecting it back into a geothermal reservoir.
2. Description of Prior Art
The solubility of most ions in solution decreases with a decrease in temperature or pressure of the solution. If dissolved ions are present near their saturation concentration in the solution, a slight reduction in the temperature or pressure of the system can result in precipitation of a portion of these ions, which often combine and deposit as a scale on any solid surface with which they come into contact, such as the vessel or conduit in which the solution is confined.
One example of such a solution is a liquid stream containing hot water which is passed through a conduit in an industrial operation under conditions, such as a lowering of the pressure, which flash at least a portion of the hot water to steam. If the hot water is a brine containing appreciable amounts of dissolved salts, this flashing is often accompanied by the formation of scale on the surfaces of the conduit contacted by the fluid stream. Scale deposits tend to build up over a period of time and restrict further fluid flow through the conduit requiring either operation at a reduced flow rate or an increase in the amount of power used to move the fluid through the conduit. In extreme cases, the conduit can become completely plugged with scale and the industrial operation must be shut down for maintenance. Industrial operations for generating steam power often are hampered by the buildup of scale deposits caused by flashing of hot water containing dissolved salts.
Geothermal steam and hot brines are found in naturally occurring, large subterranean reservoirs in many regions of the world. When hot geothermal fluids can be used to generate electric power, pressurized geothermal water or brine, having a temperature above about 400.degree. F., is flashed to a lower pressure and the steam generated thereby drives a steam turbine in combination with an electric generator. However, formidable problems are generally encountered in handling and disposing of large amounts of heavily contaminated and frequently highly saline geothermal liquids. Consequently, production of electricity from geothermal waters on a commercial scale has been difficult and costly to achieve.
One of the most serious problems encountered in using hot geothermal liquids for producing electric power results from scaling of the equipment used to confine and contain the liquid. Because geothermal liquids have usually been confined in subterranean reservoirs for extraordinarily long periods of time at elevated temperatures, large amounts of minerals are leached from the reservoirs into the brine. Typically, salts and oxides of heavy metals, such as lead, zinc, iron, silver, cadmium and molybdenum, are found in geothermal brine. Other more common minerals, such as calcium and sodium, are also dissolved in the brine, as are naturally occurring gases, including carbon dioxide, hydrogen sulfide and methane. An especially troublesome component of the hot brine may be silica, which is found in large concentrations in the form of silicic acid oligomers. These tend to precipitate out at almost every stage of brine processing, either as substantially pure silica or as a tightly adherent iron-silica scale. If this scale is not removed, or prevented from forming, serious problems can arise with conduit plugging. Even when the brine has completed its passage through the plant, it typically retains some small amount of silica, which if not removed, will eventually result in the plugging of the injection wells used to return the now cooled brine to the geothermal field.
Various proposals have been made to decrease the scale formation in equipment used in producing and handling geothermal brine. In "Field Evaluation of Scale Control Methods: Acidification," by J. Z. Grens et al, Lawrence Livermore Laboratory, Geothermal Resources Council, Transactions, Vol. 1, May 1977, there is described an investigation of the scaling of turbine components wherein a geothermal brine at a pressure of 220 to 320 p.s.i.g. and a temperature of 200.degree. to 230.degree. C. (392.degree. to 446.degree. F.) was expanded through nozzles and impinged against static wearblades to a pressure of 1 atmosphere and a temperature of 102.degree. C. (215.degree. F.). In the nozzles, the primary scale was heavy metal sulfides, such as lead sulfide, copper iron sulfide, zinc sulfide and cuprous sulfide. Thin basal layers of fine-grained, iron-rich amorphous silica promote adherence of the primary scale to the metal substrate. By contrast, the scale formed on the wearblades was cuprous sulfide, native silver and lead sulfide in an iron-rich amorphous silica matrix. When the brine which originally had a pH of 5.4 to 5.8 was acidified with sufficient hydrochloric acid to reduce the pH of the expanded brine to values between 1.5 to 5.0, scaling was eliminated. However, such acidification of hot brines increases the corrosion of the brine-handling equipment to such levels that it often defeats the purpose of using acid for scale control.
It is known to recover metal values and salts from brine, such as geothermal brine produced from a subterranean reservoir. U. S. Pat. No. 4,127,989 to Michelson discloses a method in which brine is pressurized and maintained above the bubble point pressure and thereafter a precipitating agent, such as a soluble sulfide, is added to the brine to enhance formation of insoluble metal sulfide precipitates. Soluble salts and metal values are recovered from the brine effluent after the hot brine has been processed to recover energy therefrom. Silver sulfides are among the mineral values recovered by this process.
While the aforementioned treatments have met with some success in particular applications, the need exists for a further improved treating process to better control the scaling process and to efficiently recover the mineral values in geothermal brines containing silver and/or other precious metals.
Accordingly, it is the object of this invention to provide an improved method for recovering valuable precious metal-containing scales from clarified geothermal brines, inhibiting the overall precipitation of scale, particularly iron-silicate scale, inhibiting corrosion, and polishing the fluid so as to prevent the transport of residual silica and other particulate material suspended therein to an injection well.
Other objects, advantages and features of the invention will be apparent from the following description, drawings and appended claims.